1. Technical Field
A method for fabricating a semiconductor device is disclosed. In particular, an improved method for fabricating a semiconductor device is disclosed which provides a difference in junction depth by performing at least one implant process for forming an LDD region in a MOSFET having a thin gate oxide in a high speed device having a salicide (self-aligned silicide).
2. Description of the Related Art
In general, the most important function of a transistor of a semiconductor circuit is a current driving function. A channel width of a metal-oxide-semiconductor field effect transistor (MOSFET) is adjusted in consideration of the current driving function. In the most widely-used MOSFET, an impurity-doped polysilicon layer is used as a gate electrode, and a diffusion region formed by doping an impurity on a semiconductor substrate is used as a source/drain region.
A buried channel is formed in a positive metal-oxide-semiconductor field effect transistor (PMOSFET) which uses an N+ doped polysilicon gate electrode in a complementary metal-oxide-semiconductor field effect transistor (CMOSFET). Here, because a negative metal-oxide-semiconductor field effect transistor (NMOSFET) with a channel on its surface and the PMOSFET have different threshold voltages, there are various restrictions in design and fabrication of the device.
That is, in the CMOSFET using a dual gate electrode, the dual gate electrodes are formed by ion-implanting N-type and P-type impurities twice. Therefore, a photolithography process should be performed twice, and this complicates the fabrication process. Accordingly, the device is easily contaminated due to a wet process, and thus the process yield and reliability of the devices are reduced.
FIGS. 1A through 1C are cross-sectional views illustrating sequential steps of a conventional method for fabricating such a semiconductor device which is an example of a MOSFeT having a thin gate oxide film.
First, referring to FIG. 1A, a field oxide 11 defining an active region is formed on a semiconductor substrate 10. A thin gate oxide 12 and a polysilicon layer (not shown) are formed on the semiconductor substrate 10. Thereafter, the polysilicon layer is etched using a gate electrode mask as an etching mask, to form a gate electrode 13. An LDD (lightly doped drain) region 14 is formed by ion-implanting a low concentration impurity to the semiconductor substrate 10 at both sides of or around the gate electrode 13. An insulating film spacer 15 is formed at side walls of the gate electrode 13.
As shown in FIG. 1B, a first source/drain region 16 is formed by ion-implanting a high concentration impurity to the semiconductor substrate 10 at both sides of or around the insulating film spacer 15. Thereafter, a second source/drain region 17 is formed by implanting a dopant having a high diffusion ratio at a low dose.
As shown in FIG. 1C, a silicide layer 18 is then formed on the surfaces of the gate electrode 13, and the semiconductor substrate.
However, the conventional method for fabricating the semiconductor device has a limit due to a shallow junction region resulting from miniaturization of the device. Specifically, the depth of the junction region is increased by the ion implant process for forming the silicide layer 18, which influences the LDD region 14 due to the close proximity of the silicide layer 18 to the LDD region 14 as shown in FIG. 1C. Further, when the silicide layer 18 is formed deeply along the rim of the field oxide, a leakage current is considerably increased in the junction region adjacent to the field oxide 11. Still further, as the height of the field oxide is decreased during subsequent processes, the leakage current increases.
Accordingly, a method for fabricating a semiconductor device is disclosed which can prevent an increase of the junction leakage current and which can improve the process yield and reliability, by forming a deep junction near the field oxide which does not influence the channel region of a MOSFET where a gate oxide is thin (hereinafter referred to as xe2x80x9ca core devicexe2x80x9d), by partially exposing a source/drain region adjacent to the field oxide to a photolithography process for forming an LDD region of an input/output device between the core device and a MOSFET where the gate oxide film (hereinafter referred to as xe2x80x9ca input/output device) is thick in a CMOS fabrication process, and by ion-implanting an impurity thereto at the same time an ion implant process is carried out for forming the LDD region of the input/output device region.
A disclosed method for fabricating a semiconductor device comprises: forming a field oxide defining an active region on a semiconductor substrate having a central core device region and a peripheral input/output device region; forming a gate oxide on the core device region; forming a gate electrode on the gate oxide; forming a first LDD region by ion-implanting a low concentration of impurity ions to the input/output device region of the active region; forming a photoresist film pattern over the gate electrode and on the sides of the gate electrode, the photoresist film pattern reaching from an end of the gate oxide to a part of input/output device region spaced a determined distance from the gate electrode or gate oxide; forming a second LDD region deeper than the first LDD region by implanting a low concentration of impurity ions by using the photoresist film pattern as an ion implant mask; removing the photoresist film pattern; forming an insulating film spacer on side walls of the gate electrode; forming a deep source/drain region and a shallow source/drain region by implanting a high concentration impurity ions to the input/output device region of the active region of the semiconductor substrate at least once, by using the insulating film spacer as an ion implant mask; and forming a silicide film on the gate electrode and the source/drain regions.
A novel semiconductor device made in accordance with the methods disclosed herein is also disclosed.